Such cassettes have a thin cassette housing with a circular film disc having a notched periphery and a slotted central hub-receiving opening. The film disc is rotatably mounted in a film chamber in the housing. In prior art camera designs, the film disc central opening fits over a motor driven hub projecting from the rear vertical wall of the film chamber.
A common design objective in electric motor driven cameras is rapid film advance and shutter recocking between successive exposures, in order that a rapid series of sequential pictures may be obtained. Because of the extremely close interframe spacing on the normal Kodak disc film format, the film advance must be very precisely regulated so as to prevent overlapping exposures on the film disc. Thus, not only is the film advance operation preferably rapid, but the precision of the advance must be held to very low error. This precise advancement has caused design problems in the prior art disc film cameras, which drive the film disc through a rotatable hub as desribed, requiring mechanically and electrically complex solutions.
For each depression of the shutter release button, the motor must rotate the drive hub an angular amount corresponding to a single frame advancement, while simultaneously cocking the shutter mechanism. To terminate the film advance drive, in some prior art cameras an end-of-frame sensing finger is provided protruding from the rear film chamber wall. The finger is spring loaded and positioned to engage one of the peripheral notches on the film disc which is disposed between each succesive pair of film frames. As the finger slips into a notch, an associated switch is actuated to terminate a motor driven film advancement operation.
Because such switches are necessarily very lightly loaded and have limited travel, they pose not only a manufacturing problem insofar as initial adjustment is concerned, but also tend to be unreliable. Even if the end-of-frame switch is properly adjusted, a residual problem still remains with respect to terminating the motor drive by using such a switch, since simply terminating the drive to the electric motor does not necessarily stop the drive train immediately.
Drive termination could also in principle be achieved by employing a notch-engaging element to lockingly engage the periphery to arrest it in position at the end of a frame advance operation. If film tearing is to be avoided, however, either the hub drive train must stop instantly upon such engagement, or alternatively some form of slip clutch must be inserted to allow a measure of slippage to allow the motor to come to a stop. In the latter case, since the film is in the form of a thin planar disc, an inherently fragile structure, an extremely sensitive clutch would have to be devised. Whether a clutch of requisite sensitivity would remain in proper adjustment over a substantial period of time is questionable. In any case, it would introduce additional complexity to the system.
Alternatively, one would have to make provision for virtually instantaneous motor stoppage, which is not easy to achieve economically in a disc camera application. Moreover, whichever approach is employed, provision would have to be made for disengaging the element from the notch before initiating the next film-advancing rotation, again adding to the complexity of the system.
Another limitation of the aforementioned type of film advance system is that because the film disc of these prior camera are driven from their hub-receiving center openings, a complex multi-layer gearing system is necessary in order to properly gear down the necessarily small and therefore high speed low-torque motors customarily employed to actuate such systems, the small motor size being dictated by space limitations within the camera. Since the output of the gear train drives the film disc only 1/15 of a revolution per frame advance, and since the camera mechanism must actuate a complete resetting of internal switching as well as recocking of the shutter for each frame advance, an intermediate power take-off is necessary at some point in the gear train to actuate this mechanism. These prior art cameras employ a series of camming surfaces on an intermediate gear to achieve these functions. Since many gears are employed, the train is necessarily characterized by excessive space and thickness requirements. To obtain an adequately thin drive system, the procedure employed in such prior art cameras is to mount the power train directly to the side of the film chamber wall remote from the film chamber, and to mount the associated shutter and switch sensing elements on a separate board engagingly disposed over the aforementioned intermediate gear. Not only is the component arrangement on the board quite complex, but the arrangment of parts on the rear camera wall is such as to make it difficult to assemble and disassemble the parts during manufacture or repair of the camera.
An alternative approach to the film advancing problem is disclosed in U.S. Pat. No. 4,382,668, which discloses the use of a stepping motor directly engageable with the hub of the cassette, the motor advancing a fractional rotation with each pulse supplied thereto, so as to advance the film. Such motors are, however, expensive and of rather large diameter, thus occupying a relatively large central area of the drive system so as to preclude use of these portions for related features of the system, e.g. shutter actuation and cocking and similar operations.
There are also disclosed in the art a variety of other cassette indexing systems. For example, U.S. Pat. No. 4,194,822 discloses a cassette having a specially designed hub featuring a series of indentations disposed thereon, the indentations being sequentially engaged by a resilent arm to rotate the disc stepwise to secure frame advance. This cassette is, however, if non-standard design in providing such a feature, and the system disclosed therein is of no relevance to the problem at hand, namely providing a rapid film advance and indexing system capable of employing the conventional widely marketed Kodak cassette, which has no such specially configured hub associated therewith. Similarly, U.S. Pat. No. 4,202,614 discloses another non-standard cassette wherein an eccentric drive pin associated with a central support spindle eccentrically engages a slot in a specially designed cassette hub to advance the film. Similar considerations apply to this approach as well.
Thus, there is a need for a vastly simplified parts arrangement in disc cameras and a film advance system which does not require precise motor shutdown to govern film advance. The present invention provides such a camera and film advance system by providing a substantially less bulky and simplified drive, and wherein most of the major elements of the drive train can be mounted on a single easily mounted and removed board so as to facilitate manufacturing assembly and servicing.
Finally, the film must be properly positioned with respect to the focal plane of the imaging lens. This in practice has mandated the use of a framing shroud extending outwardly from the rear of the rear film chamber wall of the camera, the shroud having a generally planar rectangular outer surface defining a light passage opening within its confines and configured to engage against the film disc by pressing against the same to establish a fixed film exposure plane. Pressure is applied to the opposite side of the film to press it against the shroud frame by means of a spring pressing upon the outside face of the cassette generally along the imaging axis, the flexing of a specially designed thin wall in the cassette being inwardly transmitted to press the film against the rectangular shroud frame surface.
To prevent film scratching during film advance, the prior art cameras use a disengaging linkage which removes the spring pressure during film advance so as to release the film, the pressure being reapplied at the end of each advance. U.S. Pat. No. 4,290,676 discloses a representative prior art system for accomplishing this result. The elimination of such system complexity would be a highly desirable feature in such cameras. The present invention provides such a feature.